The present state of the arts are defined and illustrated by many disclosures with respect to the composition, formulation, and performance of lubricant additives, lubricant systems containing solid lubricant additives, the composition and formulation of metal coatings, the composition and formulation of catalysts, and the chemistry and performance of lubricants containing solid lubricant additives, all of which bear some relevance to the invention presented herein. Those disclosures employed as references in this patent application are listed hereinafter.
The references, other than United States Patents, are presented as follows:
L. L. Cao, Y. M. Sun, and L. Q. Zheng, "Chemical Structure Characterization of Boundary Lubrication Film Using X-ray Photoelectron Spectroscopy and Scanning Auger Microprobe Techniques," Wear, 140 (1990), pp 345-357.
Harold Shaub, John Pandosh, Anne Searle, and Stan Sprague, "Mechanism Studies with Special Boundary Lubricant Chemistry," Society of Automotive Engineers, Paper 952475, 1995.
Hal Shaub, John Pandosh, Anne Searle, Stan Sprague, and Martin Treuhaft, "Engine Durability, Emissions and Fuel Economy Studies with Special Boundary Lubricant Chemistry," Society of Automotive Engineers, Paper 941983, 1994.
Keith Perrin, John Pandosh, Anne Searle, Hal Shaub, and Stan Sprague, "Radioactive Tracer Study of Start-Up Wear Versus Steady-State Wear in a 2.3 Liter Engine," Society of Automotive Engineers, Paper 952474, 1995.
The United States Patents which bear particular relevance or are of significant interest with respect to the present patent application are singled out and are cited. See U.S. Pat. Nos. 2,230,654; 2,510,112; 2,993,567; 3,194,762; 3,247,116; 3,314,889; 3,432,431; 3,493,513; 3,505,229; 3,536,624; 3,567,521; 3,592,700; 3,607,747; 3,636,172; 3,640,859; 3,723,317; 3,806,455; 3,909,431; 3,933,656; 3,969,233; 4,029,870; 4,036,718; 4,127,491; 4,224,173; 4,252,678; 4,349,444; 4,465,607; 4,484,954; 4,500,678; 4,584,116; 4,615,917; 4,657,687; 4,770,797; 4,803,005; 4,834,894; 4,857,492; 4,859,357; 4,888,122; 4,892,669; 5,009,963; 5,160,646; 5,350,727; 5,373,986; 5,447,896; 5,460,661.
Properly designed and finished metal wear surfaces exhibit minute ridges or projections which are commonly referred to as "asperities." The asperities are a natural consequence of the metal cutting, machining, and finishing processes. Furthermore, where such wear surfaces are intended to be lubricated with liquid or semisolid lubricants, it is presently a generally accepted belief that the extension of the asperities needs to be approximately 0.1 micrometers (4 micro inches), in order to retain sufficient amounts of liquid or semisolid lubricants to protect against wear. In those cases where the wear surfaces are appropriately designed and are lubricated with liquid or semisolid lubricants, the lubricants will interpose a film between the adjacent wear surfaces and tend to protect against wear. In those instances where the liquid or semisolid film is constantly maintained on the wear surfaces, while the mechanisms being lubricated are in operation, the lubrication regime is referred to as "hydrodynamic lubrication." Hydrodynamic lubrication with properly designed lubricant systems can provide very reliable wear protection for the lubricated wear surfaces. However, in order to achieve a high level of reliability, the liquid or semisolid lubricant film must remain interposed, continuous, and of sufficient thickness between the adjacent wear surfaces, so that direct contact of the wear surfaces is prevented, or at least minimized. The appropriate design of the liquid or semisolid lubricants in providing wear protection will be predicated upon the duty with which the lubricants must contend. The duty may be determined as a function of the adjacent wear surface materials, clearances, the load imposed on the wear surfaces, the relative speed of the wear surfaces, temperatures, pressures, and other related environmental conditions.
In those cases where the wear surfaces requiring lubrication are not submerged in the lubricants, the design of the mechanisms frequently are such that lubrication is dependent upon one or more of the moving wear surfaces dragging, driving, or forcing lubricants into the areas of potential wear. The period that exists following the initial relative movement of the wear surfaces, and prior to the establishment of hydrodynamic lubrication, is referred to as the "boundary period," and the lubrication characteristics that exist during this period are referred to as "boundary lubrication."
The term "boundary" is a term of art in the mechanism and lubrication design fields handed down from an old-time British researcher who once studied journal bearings. He observed that when a shaft ceased rotating within a journal bearing, and came to rest, there was metal-to-metal contact between the shaft and the journal bearing. Thereafter, when the shaft was allowed to begin rotation again, the researcher observed that there was a "boundary period" before the lubricant applied to the journal bearing began to form a film and establish hydrodynamic lubrication.
The term "boundary lubrication," which has its roots in the study conducted by this British researcher, has been used within the Lubrication Industry ever since that time. The term is used to denote the character of lubrication that takes place with respect to adjacent wear surfaces in the time period from start-up until the time at which a continuous film of lubricant is established to effect hydrodynamic lubrication. It also denotes the character of lubrication that takes place from the time of slow down to the point where the lubricant film is lost and relative movement of the adjacent wear surfaces ceases. Boundary lubrication frequently is also associated with mechanisms which are subject to acceleration, including the acceleration due to the abrupt and rapid change of direction while in operation. Presently, the term "boundary lubrication" is often times used to refer to a regime of lubrication that is not entirely hydrodynamic.
The goal of appropriately designed mechanisms, and the programs designed to lubricate those mechanisms, is to establish and maintain a lubricant film between the adjacent wear surfaces of sufficient thickness so as to avoid or at least minimize the contact between the wear surfaces. In addition, it is the goal to minimize the energy required to move the adjacent wear surfaces in the environment of the lubricants. These goals are presently best achieved in the hydrodynamic regime of lubrication by those skilled in the arts of mechanism and lubrication design, by employing lubricants with the lowest possible viscosity permissible, wherein the separation of the adjacent wear surfaces is continuously or satisfactorily maintained.
Unfortunately, all mechanisms requiring lubrication cannot be designed so that they are maintained in a hydrodynamic lubrication regime. This is so for a variety of reasons, such as the reciprocating, or irregular motion of the mechanisms, which results in irregular rates of movement from zero to some maximum value and back to zero all in one cycle of motion. In addition, all lubricated mechanisms must start and stop operation altogether, from time-to-time, which functions produce unsteady state conditions during which periods it is difficult, if not impossible, to maintain hydrodynamic lubrication. Therefore, even though mechanisms may be optimally designed to operate with state of the art hydrodynamic lubrication most of the time, it is probable that the same mechanisms are caused to experience boundary lubrication conditions at various times in their motion or operational cycle, but it is highly probable that the mechanisms will be subjected to boundary lubrication conditions at certain times during their operational life span.
Those knowledgeable of mechanism and lubrication design and operation have long recognized that mechanisms requiring lubrication are prone to suffer a much greater rate of wear during periods of boundary lubrication than during periods of hydrodynamic lubrication, within the present state of the mechanism and lubrication design arts. The relatively high rate of wear during periods of boundary lubrication has been determined to be due to the inordinately high incidence of direct contact between the adjacent wear surfaces. This wear surface contact is most often occasioned by the loss of lubricants due to the mechanisms' acceleration. The acceleration results from simple start-up or shut-down or from rapid change in speed and rapid change in directional movement, such as centrifugal motion, sinusoidal motion, reciprocating motion, etc. This propensity for the loss of lubricant may be exacerbated by decreased lubricant viscosities at elevated operating temperatures, and by the simple gravity drainage of the lubricants away from the wear surfaces of the mechanisms during operation and following shut-down.
Based on the single consideration of minimizing energy dissipation, during hydrodynamic lubrication periods, the state of the lubrication art would mandate the use of the lowest possible viscosity liquid lubricants, that would allow the adjacent wear surfaces to hydroplane past one another, avoiding all direct contact. However, because of the inevitable periods of boundary lubrication, the relatively low viscosity liquid lubricants are known to be inadequate, without other additives, to also minimize the mechanisms' rates of wear. The present state of the art solution to this lubrication dichotomy is to employ relatively low viscosity liquid lubricants with solid lubricants suspended therein, or preferably with finely divided solid lubricant particles combined with liquid or semisolid lubricant bases, and do so in such a manner as to create colloidal systems, stable under all operating conditions.
In addition to the large number of laudable liquid and semisolid lubricants which are presently known to the lubricant art, there are solid lubricants which distinguish themselves by virtue of their relatively low coefficients of friction along with their special structural or special chemical characteristics, or both. Some of the solid lubricants which have found favor, exhibit laminar lattice structures, such as molybdenum disulfide and graphite. These structural features result in relatively low coefficients of friction, when these materials are utilized in lubricant applications. Other solid lubricants such as the various organic polymers, compounds of ethers, compounds of fatty acids and carbon, calcium, barium and lithium fluorides also exhibit relatively low coefficients of friction. In addition, these solid lubricants generally are less influenced than liquid lubricants by the adverse effects of temperature changes, in that they tend to not readily drain or be thrown from the wear surfaces being lubricated. Furthermore, in many cases solid lubricants are adsorbed, absorbed, or chemically bonded to the wear surface, frequently providing satisfactory lubrication when and where liquid lubricants alone would not do so.
The appropriately designed lubricant systems, containing solid lubricants, shall have the solid lubricants evenly distributed throughout the lubricant base media, again, preferably in stable colloidal systems, and the solid particles shall be sufficiently small as to pass readily through all of the lubrication galleries and filters, and easily gain entry to all of the interstices of the mechanisms, which the lubricant systems are designed to lubricate.
The solid lubricants generally employed as additives to liquid or semisolid base lubricants commonly pass through a series of steps whereby they are first adsorbed on the wear surfaces of the mechanisms being lubricated. Thereafter, the solid lubricants are generally absorbed within the wear surfaces, and in some cases the solid lubricants ultimately react and chemically bond to the wear surfaces to form persistent, durable films, which exhibit relatively low coefficients of friction. These three steps are deemed to be part of the comprehensive bonding reaction herein. At such time as the solid lubricants have bonded to the wear surfaces, they are expected to provide adequate lubrication during boundary lubrication periods, sufficient to prevent or reduce wear.
Some of the solid lubricants presently in service and/or proposed for service are: polytetrafluoroethylene (PTFE); Teflon.RTM. (PTFE); perfluoropolyether oxide; ethylene polymers; propylene polymers; fluorophenylene polymers; perfluoropolyether; polyol monoesters of fatty acids; amides of fatty acids; sulfurized fats and esters; molybdenum sulfur compounds; metallic soaps of fatty acids; graphite; carbon fluoride; carbon fluoride chloride; barium fluoride; calcium fluoride; and lithium fluoride. It is probable that not all of these solid lubricants are capable of progressing through the three steps cited above to reach the state where the solid lubricants are bonded to the wear surfaces. In fact, solid lubricants can be divided conveniently into two groups: unbonded solid lubricants and bonded solid lubricants.
The unbonded solid lubricants are sometimes directly applied to the surfaces to be lubricated, usually in the form of a powder, and adhere thereto by some degree of mechanical or molecular action. However, the solid lubricants in this category are, by definition, not physically or chemically bonded to the surfaces being treated in such a manner. Consequently, the properties of the solid lubricants and the fact that they are adherent but unbonded will generally serve to define the performance characteristics for any specific application. Since there is no bonding of the solid lubricants to the surfaces, in the case of unbonded solid lubricants, the potential exists, particularly in load-bearing applications, that such lubricants will be extruded from between the adjacent load-bearing wear surfaces and will not remain in position to provide the desired lubrication performance for any significant period of time. For this reason unbonded solid lubricants are considered to be useful for only nonload-bearing applications or applications where "non-stick" properties are being sought, for example cookware surfaces, cling, and stain resistant surfaces, etc.
The bonded solid lubricants are, by the definition employed herein, attached to the desired wear surfaces, generally by virtue of first adsorption, then absorption, followed thereafter by a chemical bond between the solid lubricants and the wear surfaces. The bonding often times can be effected and accelerated, or both, by the use of adhesives, binders, elevated temperatures, and other materials and techniques in appropriate applications. Generally bonded solid lubricants will present different lubrication characteristics, than those lubrication characteristics exhibited by the same solid lubricants prior to the bonding reactions. However, once the bonded lubricants are firmly affixed to the wear surfaces, they will be more persistent and much less likely to be displaced under load-bearing conditions than the unbonded lubricants. The bonded solid lubricants, in almost every case, will present a bonded interface with the lubricated wear surfaces, which will be very shallow, and as a consequence, the bonded lubricants will tend to disappear as the surfaces are subjected to unavoidable wear action, if the solid lubricants are not otherwise continuously replenished.
Irrespective of the mode of lubrication, it is easy to recognize that even under the best of conditions some wear is likely to take place with respect to adjacent wear surfaces, within the present state of the mechanism and lubrication design arts. It is acknowledged that many of the lubricant systems containing solid lubricants are and can be very effective, if the lubricant systems are maintained in good functional condition, and the lubricants are caused to immediately be self-replenishing, if and when removed from the lubricated wear surfaces.
However, it is known by one skilled in the art, that the solid lubricants contained in state of the art lubricant systems, which are subject to bonding with the wear surfaces, are very slow to effect the bond reactions. This is a significant issue, which in fact serves to diminish or maybe even negate the value of these otherwise meritorious lubricant systems in specific applications.
There is extensive literature available to confirm the function and the merits of various solid lubricants containing lubrication systems, including many of the United States Patent references cited herein. In most every case disclosed in the literature, the lubrication qualities and the wear resistance values measured and presented were derived with respect to newly formed solid lubricant films. Furthermore, general familiarity with the chemistry and physics principles involved would tend to validate the observed and reported results. However, in the actual operation of mechanisms lubricated with lubrication systems containing solid lubricants, it is inevitable that eventually both the unbonded and the bonded solid lubricants will disappear due to extrusion, erosion, corrosion, abrasion, scraping, galling, grinding, volatilization, and normal wear unless they are otherwise continuously replenished. Insofar as more solid lubricants are available in the lubricant systems, it is reasonable to expect that the solid lubricants will reestablish themselves, by again adhering or bonding to the exposed wear surfaces. However, it is known that the adhering or bonding reaction generally takes place at a relatively slow pace in the present state of the art lubricant systems. As a matter of fact, it is not unreasonable to expect that the wear surfaces subject to being lubricated with the solid lubricants are devoid of such lubricants a great deal of the time, perhaps more often than not. Nevertheless, the presence of solid lubricants in the state of the art lubricant systems generally produces a lower rate of wear than those lubricant systems with no solid lubricants present.
One of the most popular solid lubricant additives is polytetrafluoroethylene ("PTFE") which is the subject of U.S. Pat. No. 2,230,654. Since the time of U.S. Pat. No. 2,230,654, PTFE has been recognized to be a bondable solid lubricant having superior lubricating properties, primarily because of its exceptionally low coefficient of friction, and its apparent penchant to resist the adherence of other materials. Furthermore, PTFE is highly resistant to most forms of chemical attack.
Based on the research work disclosed earlier by L. L. Cao, et al., it was determined that metallic wear surfaces treated with PTFE resulted in a bonded lubrication film that could be qualitatively divided into four layers, including the outermost layer of PTFE. In fact, an oil containing PTFE was subjected to boundary lubrication conditions and when the test was completed the contacting surfaces of the metal test specimens were analyzed using X-ray Photoelectron Spectroscopy and a Scanning Auger Microprobe. The chemical state of the fluorine in the bonded boundary reaction film was shown to display four different chemical structures. The chemical structures and the related binding energies are shown in TABLE 1.
TABLE 1 ______________________________________ Binding Energy, Description Chemical Structure Designation eV ______________________________________ 1. Outermost Layer (--CF.sub.2 --CF.sub.2 --) Polytetrafluoro- 689.72 ethylene 2. Second Layer In (--CFH--CFH--) Polydifluoro- 688.50 ethylene 3. Third Layer In (--CFH--CH.sub.2 --) Polymonofluoro- 687.45 ethylene 4. Metal Surface FeF.sub.x (x = 2 & 3) Ferrous & Ferric 684.42 Fluoride ______________________________________
It was clearly established that a multilayered boundary lubricant reaction film, with the structural layers cited above, was formed on the metallic surface. The outermost or first layer was composed of a film of PTFE. The second layer was composed of a mixed reaction film, containing a mixture of the chemical structures shown as Items 2, 3, and 4 above. The third layer exhibited a chemical structure in which there was a paucity of fluorine with respect to the second layer. The deepest layer consisted primarily of ferrous and ferric fluoride, along with some microparticles of PTFE. It is evident from the binding energy figures that each of the bonded layers was tightly bound, with the outermost layer exhibiting the greatest binding energy. The innermost layer, or the fourth layer was clearly reacted and had become part of the metallic matrix, even though its binding energy was determined to be slightly less than the other three layers.
The researchers, L. L. Cao, et al., concluded that, "under boundary lubrication, PTFE microparticles not only mechanically reduce friction, but also take part in the chemical reaction and form a multilayer structure of fluorine compounds which play an important role in antifriction and antiwear."
Those skilled in the arts of mechanism and lubrication design, and other students of engine wear behavior have long recognized that engines are prone to suffer a much greater rate of wear during start-up, as opposed to the rate of wear suffered during operation following start-up, with the present state of conventional engine lubricants. The relatively high rate of wear during start-up is attributable to the inordinately high incidence of metal-to-metal contact between the engine's adjacent wear surfaces. The inordinately high incidence of contact is occasioned by inadequate lubrication, sometimes simply due to gravity drainage of the lubricating oil away from the wear surfaces and into the engine oil pan prior to start-up.
Those engine parts having wear surfaces subject to submergence in the lubricating oil are lubricated by a hydrodynamic process which allows the adjacent wear surfaces to "hydroplane" on a film of oil, and hence, it is theoretically possible to almost completely avoid contact between the adjacent wear surfaces. Such are the conditions under which the main crankshaft bearings, the lower connecting rod bearings, and other wear surfaces submerged in the engine oil are allowed to operate. However, other wear surfaces in the engine, not submerged in the lubricating oil, are subject to boundary lubrication and/or some combination of boundary and hydrodynamic lubrication. In the case of boundary lubrication, such as that experienced by the engine piston rings, the wear surfaces are exposed to direct metal-to-metal contact and hence the rate of wear shall be very dependent upon the presence or absence of the appropriate lubricant at the points of potential contact. Realization of the manner in which lubricants function in a typical engine, and/or other mechanisms having adjacent wear surfaces, serves to emphasize the need for improved lubricant systems to enhance boundary lubrication, and consequently to reduce drag and mitigate wear.
The contemporary state of the art lubricant system using PTFE is dependent on the adherence of the PTFE on the lubricated wear surfaces, followed thereafter by metal-to-metal contact of the asperities to generate exceedingly high localized temperatures. These exceedingly high localized temperatures are deemed to be required to promote a chemical bonding reaction between the PTFE and the metallic wear surfaces being lubricated. The obvious shortcoming of this process is that the metal-to-metal contact relied upon to generate the necessary localized high temperatures deemed necessary to promote the bonding reaction, is the same metal-to-metal contact which serves to physically diminish and remove any preexisting lubricant film. That is to say the mechanical metal-to-metal collisions of the adjacent metal wear surfaces with the attendant erosion, corrosion, abrasion, scraping, galling, grinding, normal wear, and lubricant volatilization effects are probably almost as efficient in removing the lubricant film as they are in building the lubricant film. It is because of this inescapable logic and because of the test data presented in the cited references, that it is reasonable to conclude that wear surfaces lubricated with present day state of the art lubricant systems, such as the one discussed herein, are exposed without the benefits of the lubricant film, perhaps as often as they are protected by the film.
Based on the references cited herein and others, there is little or no doubt that many of the presently disclosed lubricant systems, wherein solid lubricants are contained in liquid or semisolid lubricant base materials, have proven to be more effective lubricant systems than the lubricant base materials without the solid lubricant additives. In many instances it was disclosed that the measured boundary lubrication wear rates were reduced in the order of one half, with the application of the typical contemporary lubricant system, cited herein, which system contained PTFE and other additives.